Update comments in examples (#457)

It's seems not that important, but this helps the new user understand the code.

examples/rfid_default_keys/rfid_default_keys.ino

@@ -119,7 +119,7 @@ bool try_key(MFRC522::MIFARE_Key *key)
  * Main loop.
  */
 void loop() {
-    // Look for new cards
+    // Reset the loop if no new card present on the sensor/reader. This saves the entire process when idle.
     if ( ! mfrc522.PICC_IsNewCardPresent())
         return;